In January 2004, two robotic geologists named Spirit and Opportunity landed on opposite sides of the red planet. With far greater mobility than the 1997 Mars Pathfinder rover, these robotic explorers have trekked for miles across the Martian surface, conducting field geology and making atmospheric observations. Carrying identical, sophisticated sets of science instruments, both rovers have found evidence of ancient Martian environments where intermittently wet and habitable conditions existed.

During the rovers' landings, parachutes deployed to slow the descending spacecraft, rockets fired to slow them still more just before impact, and airbags inflated to cushion their landing. After bouncing and rolling to a halt, a protective structure of petals opened, brought the landers to an upright position, and provided a platform from which the rovers drove onto the Martian surface.

Since leaving their landing sites, the twin rovers have sent more than 100,000 spectacular, high-resolution, full-color images of Martian terrain as well as detailed microscopic images of rocks and soil surfaces to Earth. Four different spectrometers have amassed unparalleled information about the chemical and mineralogical makeup of Martian rocks and soil. Special rock abrasion tools, never before sent to another planet, have enabled scientists to peer beneath the dusty and weathered surfaces of rocks to examine their interiors.

Each rover weighs nearly 180 kilograms (about 400 pounds). Two and a half years after landing, both rovers are still working and have far exceeded their initial 90-day warranties on Mars.

Opportunity's study of "Eagle" and "Endurance" craters revealed evidence for past inter-dune playa lakes that evaporated to form sulfate-rich sands. The sands were reworked by water and wind, solidified into rock, and soaked by groundwater. Opportunity is examining more sedimentary bedrock exposures along a route leading from "Endurance" to "Victoria Crater," where an even broader, deeper section of layered rock is likely exposed that could reveal new aspects of Martian geologic history in Meridiani Planum.

While Spirit's initial travels in Gusev Crater revealed a more basaltic setting, after reaching the "Columbia Hills" the rover found a variety of rocks indicating that early Mars was characterized by impacts, explosive volcanism, and subsurface water. Unusual-looking bright patches of soil turned out to be extremely salty and affected by past water. At "Home Plate," a circular feature in the "Inner Basin" of the "Columbia Hills," Spirit discovered finely layered rocks that are as geologically compelling as those found by Opportunity and that may hold clues to a history of past water in Gusev Crater.